Ion Chromatography

What is Ion Chromatography?

Ion chromatography is used for water chemistry analysis. Ion chromatographs are able to measure concentrations of major anions, such as fluoride, chloride, nitrate, nitrite, and sulfate, as well as major cations such as lithium, sodium, ammonium, potassium, calcium, and magnesium in the parts-per-billion (ppb) range. Concentrations of organic acids can also be measured through ion chromatography.

This ion chromatography machine can analyze water chemistry. Photo by Monica Bruckner, courtesy of the Skidmore Lab, Montana State University, Bozeman.

How Does Ion Chromatography Work?

Ion chromatography, a form of liquid chromatography, measures concentrations of ionic species by separating them based on their interaction with a resin. Ionic species separate differently depending on species type and size. Sample solutions pass through a pressurized chromatographic column where ions are absorbed by column constituents. As an ion extraction liquid, known as eluent, runs through the column, the absorbed ions begin separating from the column. The retention time of different species determines the ionic concentrations in the sample.

Applications

Some typical applications of ion chromatography include:

Drinking water analysis for pollution and other constituents

Determination of water chemistries in aquatic ecosystems

Determination of sugar and salt content in foods

Isolation of select proteins

How to - Sample Collection, Preparation and Concerns

Liquid Samples:

Liquid samples should be filtered prior to evaluation with an ion chromatograph to remove sediment and other particulate matter as well as to limit the potential for microbial alteration before the sample is run. Aqueous samples should be collected using a sterile syringe or bottle rinsed three times with sample water and then filtered through 0.45um (or smaller) filters. The collection vial should likewise be rinsed three times with filtrate before being filled brim-full of sample filtrate. Samples should be stored cold until they can be processed. The minimum sample required for analysis is approximately 5mL, with no maximum limits.

Solid samples and Organic Liquids

Solid samples can be extracted with water or acid (cations) to remove ions from the sample surface. Liquid samples must also be filtered and stored cold until analysis can be performed. The minimum sample required for a solid sample is approximately 2-3 cm2 for solids, with no maximum limits.

Data Output and Analysis

This schematic diagram of an ion chromatography run depicts how elution time correlates to output peak data. Diagram courtesy of Madison Area Technical College. Copyright 2006 by the Biotechnology Project at MATC.

This ion chromatograph displays data from an anion analysis of glacial waters. Each peak represents the concentration of each anion. Image and data from Monica Bruckner.

The diagram on the upper left shows how an ion chromatograph works to output data. Each peak represents a separate ion from the sample solution. The elution time, or time it takes for the ion to move through the column, varies for each ion species as they elute from the column separately as the pH and/or ionic strength of the eluent is increased. The concentration of ions moving through the column at a particular time is represented by the height and the breadth of the peaks and can be correlated to the concentration of a particular species in the sample solution.

The graphs on the upper right display typical data output from an ion chromatography run. The upper graph shows cation concentrations and the lower graph depicts anion concentrations from dilute glacial waters. Ion concentrations can be calculated using the area under each peak, where a larger area correlates with a higher concentration of a particular ion species. Most ion chromatography machines provide software that calculates this area, which users can convert to ppm or other quantity using calibration standard solutions.

Literature

For more detailed information regarding the theory and practice of ion chromatography, please see: